E-Zine March 2006
A fieldbus is a communications network used in industrial manufacturing to collect
measurement and sensor data, and to distribute outputs to final control elements. Fieldbus is not
restricted to process control or any variety of factory automation. In general, a fieldbus is
designed to operate in the industrial environment. It also delivers power to the field devices when required.
HART was created in the late 1980's to demonstrate that it was possible to carry digital
data on the same wired network as used for 4-20 mA DC data transmissions. Compromises made to carry both
the analog current signal and digital data on the same wire resulted in a relatively slow digital data
rate (1200 bps). However, HART specification C8PSK will allow higher speed digital data transmission (9600 bps)
in a manner compatible with older HART transmitters.
HART process control field instrumentation sales exceed the sales of
equipment for the other fieldbuses. Few analog only field transmitters are being sold
by the major instrument suppliers. This occurred because HART enables the instrument
range to be set using software at any time after the instrument is manufactured using
a commonly available HART handheld communicator or computer-based configuration software.
Instrument suppliers have been offering HART instruments with wide-range
transducers to exploit this advantage. This allows the manufacturer to make fewer models to
meet the range needs of their customers. For example, analog pressure transmitters may have
been manufactured with maximum full scale ranges of 50, 100, 200, and 1000 psig. A single HART
transmitter with a maximum range of 1000 psig can meet these needs. Prior to shipment, the range
values are set with a HART terminal to meet the specifications of the user. As a result, the
manufacturer needs to make only one model for these ranges. This production volume of the model
decreases inventory and reduces manufacturing cost.
Due to the wider rangeability of HART instruments, both the supplier and
end user need to stock fewer models of field instruments to meet their range needs. This
lowers their inventory cost and makes spare parts more universal. These inventory cost
reductions have caused many users to specify only HART transmitters, even when the
digital data capability of HART is not used.
HART instruments have the capability to scale their analog output values
anywhere within the hardware limits of the transmitter. The universal parameters of the
transmitter configuration can be modified using a HART handheld communicator or another
configuration tool such as application software for a personal computer. HART configuration
depends on the form of configuration tools used and the Device Descriptions supported by the
supplier of the device. Device Descriptions that are available in the configuration tool can
be used to more fully configure the instrument and display internal data. As a result, HART
devices are able to provide multiple measurements and diagnostic capabilities.
Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control
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